Liquid crystal displays (LCDs) are typically comprised of two flat glass sheets that encapsulate a thin layer of liquid crystal material. An array of transparent thin-film electrodes on the glass modulate the light transmission properties of the liquid crystal material, thereby creating the image. By incorporating an active device such as a diode or thin film transistor (TFT) at each pixel, high contrast and response speed can be achieved to produce high resolution displays. Such flat panel displays, commonly referred to as active matrix LCDs (AMLCD), have become the predominant technology for high performance displays such as notebook computer and portable televisions.
At present, most AMLCDs utilize amorphous silicon (a-Si) TFTs. The fabrication process for a-Si typically consists of successive deposition and patterning of thin films using elevated temperature processes which result in substrate heatings to temperatures in the range of 200.degree.-450.degree. C. Because of the high registration requirement between patterning steps for these thin films, the glass substrates often require a dimensional stability (low shrinkage) in the 5-20 parts per million (ppm) range throughout this processing. Five to twenty parts per million shrinkage means, for example, 2.5-10 microns shrinkage over a substrate length of 500 mm. When greater than 5-20 ppm shrinkage occurs, registration errors will accrue between components applied subsequently in the polysilicon TFT manufacturing process.
One approach to solving this glass shrinkage problem is to make the kinetics of the densification slow compared to the time scale of the subsequent processing steps. This can be achieved by utilizing glasses which have a high strain point. One such glass, manufactured by Corning Incorporated and known as Code 1737 glass, has the highest strain point (about 666.degree. C.) currently commercially available for flat panel display substrates. This glass can be used in processing conditions of 1 hour at 450.degree. C. with negligible dimensional shrinkage in the glass.
It has long been recognized that the use of polycrystalline silicon (poly-Si) would offer certain advantages over a-Si. Poly-Si has a much higher drive current and electron mobility, thereby allowing reduction of TFT size and at the same time increasing the response speed of the pixels. It is also possible, using poly-Si processing, to integrate display drive circuitry directly onto the glass. Conversely, a-Si requires discrete driver chips which are attached to the display periphery using integrated circuit packaging techniques such as tape carrier bonding.
Unfortunately, poly-Si requires higher processing temperatures than a-Si. Poly-Si is conventionally made by depositing amorphous silicon onto a glass sheet using chemical vapor deposition (CVD) techniques, and subsequently exposing the coated glass to high temperatures for a sufficient period of time to crystallize the a-Si to poly-Si. This crystallization step is typically done at about 600.degree. C. for several tens of hours. Alternatively, rapid thermal annealing or laser crystallization can be employed, wherein a laser or other sharp temperature gradient is used to minimize heating of the glass substrate. In either case, the substrate experiences 400.degree.-600.degree. C. processing temperatures during the poly-Si crystallization process. In addition, there are commonly several other high temperature (600.degree. C.) processes following the crystallization step. Such process steps include deposition of the gate oxide, annealing of the gate oxide, and source/drain annealing.
The relatively high temperatures of the crystallization and subsequent processing steps encountered during poly-Si TFT manufacturing greatly increases the potential for glass substrate shrinkage.
One approach to solving this severe glass shrinkage problem is to anneal the glass before subjecting the glass to the AMLCD processing conditions. This serves to predensify the glass, making the subsequent compaction less. However, standard annealing processes require relatively longer times at higher temperatures. These long annealing times make processing extremely difficult and costly. In addition, at these higher temperatures, the glass has a lower viscosity. As a result, the glass substrate may incur sheet warp, sheet sag, or surface defects and imperfections during the annealing process.
It would therefore be desirable to develop a process to precompact an existing commercial glass substrate (such as Corning Incorporated code 1737 glass) and thereby enable the glass to meet the 5-20 ppm shrinkage level, even after exposure to poly-Si processing temperatures. Preferably, such a process should ideally avoid the need for both the high temperatures and long times utilized in conventional annealing operations.